Electronic control power steering device

ABSTRACT

An electric power steering apparatus ( 11 ) includes a reduction mechanism (G) for transmitting the rotation of a rotating shaft ( 28   a ) of a motor ( 28 ) to a ball screw nut ( 20 ) while reducing the speed of the rotation. The reduction mechanism includes a drive gear ( 29 ), which is coaxially connected to the rotating shaft of the motor, and a driven gear ( 16   a ), which is arranged on the outer surface of the ball screw nut. Due to the reduction mechanism, the motor may be inclined relative to a rack shaft ( 18 ).

TECHNICAL FIELD

[0001] The present invention relates to an electric power steeringapparatus that applies an assist force generated by a motor to a vehiclesteering system.

BACKGROUND ART

[0002] A rack-and-pinion steering apparatus is one type of knownsteering apparatuses for vehicles. In this type of steering apparatus, asteering wheel is connected to a pinion shaft having pinion teeth, andthe pinion shaft rotates as the steering wheel is rotated. The pinionteeth of the pinion shaft mesh with rack teeth of a rack shaft, and therack shaft moves along its axis as the pinion shaft rotates. The twoends of the rack shaft are connected to two steered wheels (frontwheels) by tie rods, respectively. The steering angle of the steeredwheels changes as the rack shaft moves in its axial direction.

[0003] An electric power steering apparatus including the knownrack-and-pinion steering apparatus has an auxiliary motor and a ballscrew mechanism for assisting the axial movement of the rack shaft. Theball screw mechanism includes a ball screw nut having a helical ballgroove in its inner circumference, a helical ball groove provided in theouter circumference of the rack shaft, and a plurality of ballsinterposed between the two ball grooves. The ball screw nut and theballs convert the rotation of the motor into axial movement of the rackshaft.

[0004] A first prior art of an electric power steering apparatus, whichincludes a reduction mechanism for transmitting the rotation of a motorto a ball screw nut while reducing the speed of the rotation, isdescribed in Japanese Laid Open Patent Publications Nos. 60-25853 and8-99643.

[0005] In the first prior art, the reduction mechanism includes two spurgears which mesh with each other. One of the spur gears is connected tothe rotating shaft of the motor, and the other one of the spur gears isfixed to a ball screw nut of a ball screw mechanism. In order totransmit the power of the rotating shaft of the motor to the ball screwnut via the two spur gears, the motor is arranged parallel to the rackshaft.

[0006] A second prior art of a steering apparatus is described inJapanese Laid Open Utility Model Publication No. 6-44674. The steeringapparatus includes a reduction mechanism including a worm and a wormwheel that mesh with each other. The worm is connected to a rotatingshaft of a motor, and the worm wheel is fixed to a ball screw nut of aball screw mechanism. In order to transmit the power of the rotatingshaft of the motor to the ball screw nut via the worm and the wormwheel, the rotating shaft of the motor is arranged perpendicular to therack shaft.

[0007] A third prior art of an electric power steering is described inJapanese Laid Open Patent Publication No. 2000-95123. The steeringapparatus includes a rack shaft, a stator having a coil that covers therack shaft, and a rotor arranged coaxially with the rack shaft. Therotation of the rotor is converted into the movement of the rack shaftby a ball screw.

[0008] In a typical vehicle, other devices such as an engine, atransmission, etc., are arranged around the rack shaft. However, in thefirst prior art, the rotating shaft of the motor can only be arrangedparallel to the rack shaft. In the second prior art, the rotating shaftof the motor can only be arranged perpendicular to the rack shaft.Therefore, the power steering apparatuses of the first and second priorarts have problems in that the motor interferes with the installation ofother devices of the vehicle and the freedom of design in determiningpositions for installing other devices is restricted.

[0009] In the third prior art, the rack shaft, the stator, and the rotorare retained in a rack housing. The rack housing, which is relativelylarge, results in problems in that the rack housing interferes with theinstallation of other devices of the vehicle (the engine, transmission,etc.) and the amount of freedom of design in determining positions forinstalling other devices is restricted.

[0010] In addition, in the first to third prior arts, fastening means,such as bolts and nuts, integrally connects a casing retaining theelectric motor to a housing, which retains a power-assist unit fortransmitting the power of the electric motor to a steering gearmechanism. Therefore, there is a problem in that the vibration and noiseof the electric motor are transmitted to the steering apparatus throughthe housing retaining the power assist unit and ultimately transmittedto the driver and the passenger compartment. In order to solve thisproblem, Japanese Laid Open Patent Publication No. 2000-350397 describesan electric power steering apparatus in which an electric motor and ahousing are connected to each other by a bushing. The bushing has ametal body covered slightly by a rubber elastic member. Thus, theelectric motor and the housing are connected to each other only by themetal body, and the transmission of vibration and noise of the electricmotor cannot be blocked sufficiently. In addition, the electric motor issupported by the elastic member in a floating manner with respect to thehousing, and rattling of the electric motor may occur due to thefloating support. When the electric motor rattles, there is a risk ofthe rotating shaft of the electric motor applying excessive force to thepower transmission shaft of the power assist unit, and therefore thepower assist unit cannot be operated smoothly.

DISCLOSURE OF THE INVENTION

[0011] It is an object of the present invention to provide an electricpower steering apparatus that increases the degree of freedom for theinstallation angle of a motor with respect to a rack shaft and a ballscrew mechanism to facilitate the arrangement of other devices of thevehicle around the rack shaft and decrease the influence of errors inassembly and dimensional accuracy of components in a reductionmechanism. It is another object of the present invention to provide anelectric power steering apparatus that blocks the transmission ofvibration and noise of an electric motor.

[0012] To achieve the above objects, one aspect of the present inventionprovides an electric power steering apparatus which transmits a drivingforce of a motor to a rack shaft via a reduction mechanism and a ballscrew mechanism to assist movement of the rack shaft in the longitudinalaxis of the rack shaft. The reduction mechanism includes a drive gearconnected to a rotating shaft of the motor and a driven gear whichmeshes with the drive gear and which is connected to the ball screwmechanism. The rotating shaft of the motor intersects the longitudinalaxis of the rack shaft at an arbitrary angle.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1A is a cross-sectional view of an electric power steeringapparatus according to a first embodiment of the present invention;

[0014]FIGS. 1B and 1C are enlarged partial views showing the powersteering apparatus of FIG. 1;

[0015]FIG. 2 is a front view showing the power steering apparatus ofFIG. 1;

[0016]FIG. 3 is a cross-sectional view of a reduction mechanism in thepower steering apparatus taken along line 3-3 in FIG. 2;

[0017]FIG. 4 is an enlarged partial view showing the reduction mechanismof FIG. 3;

[0018]FIG. 5 is an outer view showing a power steering apparatusaccording to a second embodiment of the present invention;

[0019]FIG. 6 is a cross-sectional view of the main part of the powersteering apparatus shown in FIG. 5;

[0020]FIG. 7 is an enlarged view of FIG. 6;

[0021]FIG. 8 is a plan view showing an electric power steering apparatusaccording to a third embodiment of the present invention;

[0022]FIG. 9 is a cross-sectional view of the electric power steeringapparatus shown in FIG. 8;

[0023]FIG. 10 is a cross-sectional view showing the main part of FIG. 8;

[0024]FIG. 11 is a diagram showing a modification of FIG. 10; and

[0025]FIG. 12 is a cross-sectional view of an elastic member.

BEST MODE FOR CARRYING OUT THE INVENTION

[0026] An electric power steering apparatus 11 according to a firstembodiment of the present invention will be described below.

[0027] As shown in FIG. 1A, the electric power steering apparatus 11includes a hollow rack housing 13 which is provided with two attachments12, each formed on one of the two ends of the steering apparatus 11. Thepower steering apparatus 11 is attached to a vehicle body (not shown) byfixing the attachments 12 to the vehicle body with screws.

[0028] As shown in FIG. 1B, the rack housing 13 supports a nut-retainingsleeve 16 with first and second bearings 14 and 15 so that rotation ofthe nut-retaining sleeve 16 is enabled but longitudinal movement of thenut-retaining sleeve 16 is disabled. The nut-retaining sleeve 16 has alarge-diameter portion on the end near the center S of the rack housing13 in the longitudinal direction of the rack housing 13. A nut 17 isfitted into the large-diameter portion.

[0029] A female thread is formed at the end of the large-diameterportion, and a lock screw 22 is engaged with the female thread. The nut17 is pressed against a stepped portion of the large-diameter portion bythe lock screw 22. The nut-retaining sleeve 16 and the nut 17 define aball screw nut 20. The ball screw nut 20 covers part of a rack shaft 18.

[0030] A first ball groove 17 a is formed helically in the innercircumferential surface of the nut 17. A second ball groove 18 a isformed helically in the rack shaft 18 for a predetermined length in thelongitudinal direction of the rack shaft 18. In the state shown in FIG.1A, the first ball groove 17 a faces the second ball groove 18 a at thecentral region of the second ball groove 18 a. A plurality of balls 19are received between the first ball groove 17 a and the second ballgroove 18 a in a manner that rolling of the balls 19 is enabled. Theball screw nut 20, the second ball groove 18 a, and the balls 19 definea ball screw mechanism 21.

[0031] Due to the ball screw mechanism 21, the rotation of the rackshaft 18 with respect to the rack housing 13 is disabled but-the axialmovement of the rack shaft 18 is enabled. The two ends of the rack shaft18 are respectively connected to left and right front wheels (not shown)by tie rods (not shown). In FIG. 1A, rack teeth 18 b are formed in therack shaft 18 at the right of the center S.

[0032] As shown in FIG. 2, a pinion shaft sleeve 26 is provided at theright of the center S of the rack housing 13, preferably in the vicinityof the right end of the rack housing 13. A pinion shaft 25 is rotatablydisposed in the pinion shaft sleeve 26. Thus, the pinion shaft 25 isarranged near the end of the rack housing 13.

[0033] With reference to FIG. 2, the rotation of a steering wheel Hrotates the pinion shaft 25. The axis P of the pinion shaft 25 isinclined at an angle θ1 with respect to the axis O of the rack housing13 (the rack shaft 18). In the first embodiment, the angle θ1 is about70 degrees.

[0034] As shown in FIG. 1, a pinion gear 25 a, which meshes with therack teeth 18 b, is formed on the basal end of the pinion shaft 25. Therack teeth 18 b and the pinion gear 25 a define a rack-and-pinionmechanism. Rotation of the steering wheel H rotates the pinion shaft 25and the pinion gear 25 a and moves the rack shaft 18 having the rackteeth 18 b along the axis O. Movement of the rack shaft 18 changes thesteering angle of the front wheels (not shown).

[0035] As shown in FIG. 2, a motor sleeve 27 used for attaching a motor28 is provided at a position on the left of the center S of the rackhousing 13, preferably in the vicinity of the left end of the rackhousing 13. Accordingly, the motor 28 is arranged so as to face thepinion shaft sleeve 26 across the center S of the rack housing 13. Asshown in FIG. 3, a rotating shaft 28 a of the motor 28 is inserted inthe motor sleeve 27. The axis M of the motor 28 (rotating shaft 28 a) isinclined at an angle θ2 with respect to the axis O of the rack shaft 18.In the first embodiment, the angle θ2 is about 70 degrees.

[0036] A first screw gear 29, or a drive gear, is connected to thedistal end of the rotating shaft 28 a. The first screw gear 29 isrotatably supported by third and fourth bearings 30 and 31, which arefixed to the motor sleeve 27. As shown in FIG. 1B, a second screw gear16 a, or a driven gear, which meshes with the first screw gear 29, isformed on the outer circumferential surface of the nut-retaining sleeve16 near the left end of the nut-retaining sleeve 16. The first screwgear 29 is coaxial with the rotating shaft 28 a. The first screw gear 29is in contact with the second screw gear 16 a inclined at an angle ofabout 70 degrees. The first screw gear 29 and the second screw gear 16 adefine a reduction mechanism G (screw gear mechanism).

[0037] A reference line T, which extends through the center S of therack housing 13 and which is perpendicular to the axis O is shown inFIG. 2. In the state in which the pinion shaft 25 and the motor 28 areconnected to the rack housing 13, the axis P and the axis M aresymmetric to each other about the reference line T. The pinion shaft 25and the motor 28 are arranged on the same plane. Both the angle betweenthe axis P and the reference line T and the angle between the axis M andthe reference line T are about 20 degrees. Accordingly, the distancebetween the motor 28 and the pinion shaft 25 increases as the heightfrom the rack housing 13 increases.

[0038] The rotating shaft 28 a of the motor 28 rotates in both forwardand reverse directions. The driving force of the motor 28 is transmittedto the nut-retaining sleeve 16 via the first screw gear 29 and thesecond screw gear 16 a. The ball screw mechanism 21 converts therotational torque of the nut-retaining sleeve 16 into an assist forcefor the reciprocating movement of the rack shaft 18 along the axis O.Due to this assist force, the steering force applied to the steeringwheel H is reduced.

[0039] The power steering apparatus 11 according to the first embodimenthas the advantages described below.

[0040] (1) In the reduction mechanism of the first prior art, the axesof the two spur gears can only be arranged parallel to each other. Inthe reduction mechanism of the second prior art, the axis of the wormand the axis of the worm wheel can only be arranged perpendicular toeach other. In comparison, in the first embodiment, the reductionmechanism G is the screw gear mechanism including the first screw gear29 and the second screw gear 16 a. In the screw gear mechanism, theangle θ1 between the axis of the first screw gear 29 and the axis of thesecond screw gear 16 a are set arbitrarily by adjusting the direction ofthe tooth trace and the helix angle. In other words, there is a greaterfreedom for setting the installation angle of the motor 28 with respectto the rack shaft 18 and the ball screw mechanism 21 compared to theprior arts. Therefore, the electric power steering apparatus 11facilitates the arrangement of other devices of the vehicle (the engine,the transmission, etc.) around the rack shaft 18.

[0041] (2) FIG. 4 is an enlarged view showing the meshing of the firstscrew gear 29 and the second screw gear 16 a. Due to errors in assemblyand dimensional accuracy of components in the electric power steeringapparatus 11, the first screw gear 29 may be displaced. Since thereduction mechanism G is the screw gear mechanism, even when the firstscrew gear 29 is displaced to a position shown by the two-dot chainlines, the meshing of the first screw gear 29 with the second screw gear16 a is ensured. In the first embodiment, the influence of errors incomponents and installation errors is small.

[0042] (3) In the reduction mechanism G, the reduction ratio (the ratioof the number of teeth of the first screw gear 29 to the number of teethof the second screw gear 16 a) may easily be changed. By adjusting thereduction ratio, a compact and light motor with a low output torque maybe used. This facilitates the installation of other devices. Sincecompact and light motors with a small output torque are relativelyinexpensive, the electric power steering apparatus 11 is inexpensive.

[0043] (4) The ball screw mechanism 21 of the first embodiment includesthe ball screw nut 20 having the first ball groove 17 a, the second ballgroove 18 a provided in the outer circumference of the rack shaft 18,and the balls 19 arranged between the first ball groove 17 a and thesecond ball groove 18 a. The second screw gear 16 a is provided on theouter circumference of the ball screw nut 20. Therefore, when the motor28 and the first screw gear 29 rotate, the second screw gear 16 a andthe ball screw nut 20 rotate accordingly. The rotation of the ball screwnut 20 is converted into the axial movement of the rack shaft 18 withthe assistance of the balls 19. Accordingly, since the second screw gear16 a is provided on the outer circumference of the ball screw nut 20,the driving force of the motor 28 is directly transmitted to the balls19 via the second screw gear 16 a and the ball screw nut 20 withoutloss.

[0044] (5) The second screw gear 16 a is formed directly on the outercircumferential surface of the ball screw nut 20 (nut-retaining sleeve16). Accordingly, in comparison to when the second screw gear 16 a isformed separately from the ball screw nut 20, the number of componentsof the electric power steering apparatus 11 is reduced. In addition, itis not necessary to fix a separately formed second screw gear 16 a tothe ball screw nut 20.

[0045] (6) In the electric power steering apparatuses of the first andthird prior arts, the motor is arranged at the center of the rack shaft.Accordingly, it is necessary to arrange the engine, the transmission,etc. such that they do not interfere with the motor. In comparison, inthe first embodiment, the motor 28 is arranged near an end of the rackhousing 13 and not in the vicinity of the center S of the rack housing13. Accordingly, in the vehicle including the electric power steeringapparatus 11, the arrangement of the engine, the transmission, etc. isfacilitated.

[0046] (7) The motor 28 and the pinion shaft 25 are respectivelyarranged at the two ends of the rack housing 13. Accordingly, arelatively large space is provided around the central region of theelectric power steering apparatus 11.

[0047] (8) The axis P of the pinion shaft 25 and the axis M of the motor28 are symmetric to each other about the reference line T, which extendsthrough the center S of the rack housing 13 perpendicular to the axis O.In other words, the distance from the reference line T to the axis P ofthe pinion shaft 25 is the same as that from the reference line T to theaxis M of the motor 28. Accordingly, the electric power steeringapparatus 11 has a superior balance of volume and weight.

[0048] (9) When the reduction mechanism includes a worm and a wormwheel, the reduction ratio must be greater than about {fraction (1/10)}.However, since the reduction mechanism G of the first embodimentincludes the first screw gear 29 and the second screw gear 16 a, thereduction ratio of the reduction mechanism G may be set to a value thatis less than that of the worm gear mechanism and thus has a wide settingrange.

[0049] (10) In the first embodiment, the distance between the motor 28and the pinion shaft 25 increases as the height from the rack housing 13increases. Accordingly, compared to when the distance between the motorand the pinion shaft decreases as the height from the housing increasesor when the motor and the pinion shaft are perpendicular to the rackhousing, the engine, the transmission, etc. are easily arranged abovethe rack housing 13 in the vehicle including the electric power steeringapparatus 11 of the first embodiment.

[0050] The first embodiment may also be modified as follows.

[0051] The angle θ2 between the axis M of the motor 28 and the axis O ofthe rack shaft 18 is not limited to about 70 degrees. As the angle θ2increases, that is, as the upper end of the motor 28 is positionedfurther away from the center, the space available for installing theengine, the transmission, etc. increases.

[0052] The axis P and the axis M do not have to lie along the sameplane. For example, the axis M may be rotated by a predetermined angle(for example, 90 degrees) with respect to the axis O. In such a case, itis preferred that the position of the motor sleeve 27 be changed. Themotor 28 and the pinion shaft 25 may be arranged with a relatively largedegree of freedom in accordance with the other devices that areinstalled in the vehicle.

[0053] The second screw gear 16 a may be formed separately from thenut-retaining sleeve 16 and attached to the nut-retaining sleeve 16.

[0054] An electric power steering apparatus 100 according to a secondembodiment of the present invention will now be described. As shown inFIG. 5, the power steering apparatus 100 includes a steering gearmechanism 109, which is formed based on a rack-and-pinion gearmechanism, and a power-assist unit 105, which has an electric motor 103.The steering gear mechanism 109 includes a rack shaft 106 and a piniongear 191, which is connected to a steering shaft (not shown). A ballscrew mechanism 166 is formed between the rack shaft 106 and a ball nut162.

[0055] As shown-in FIG. 6, the power-assist unit 105 includes atransmission mechanism (reduction mechanism) 101 for transmitting thepower of the motor 103 to the rack shaft 106. The transmission mechanism101 is a bevel gear mechanism including a driven bevel gear 161, whichintegrally retains the ball nut 162 with a retainer by means of a key169, and a drive bevel gear 111, which meshes with the driven bevel gear161. The motor 103 is connected to the rotating shaft of the drive bevelgear 111 by a coupling 102.

[0056] The transmission mechanism 101 of the second embodiment is anangular bevel gear mechanism in which the rotational axis of the motor103 intersects the center axis of the rack shaft 106 at a predeterminedangle, which excludes a right angle. However, the transmission mechanism101 may also be a miter gear mechanism in which the intersection angleis 90 degrees. In addition, the driven bevel gear 161 may also be formedintegrally with the ball nut 162.

[0057] In the power steering apparatus 100 of the second embodiment, thesteering gear mechanism 109 and the power-assist unit 105 are arrangedat different positions. More specifically, the pinion gear 191 and thepower-assist unit 105 are separated from each other along the axis ofthe rack shaft 106 and lie along the same plane in a generally symmetricmanner. Since the power-assist unit 105 is separated from the piniongear 191, the power steering apparatus 100 is well-balanced. Inaddition, the structure around the pinion gear 191, where variouscomponents related to the steering gear mechanism 109 are arranged in aconcentrated manner, is simplified. This enables the space around thesteering gear mechanism 109 to be used effectively. In addition, whenthe installation position of the steering wheel is changed between leftand right in accordance with where the vehicle is shipped to or forother reasons, the necessary changes in the power steering apparatus 100can be made quickly.

[0058] Next, the power-assist unit 105 will be described. As shown inFIGS. 6 and 7, the rotating shaft of the motor 103 is connected to therelatively compact drive bevel gear 111 by the coupling 102, which maybe an Oldham coupling. The drive bevel gear 111 meshes with the drivenbevel gear 161. The driven bevel gear 161 is connected to the ball nut162 by means of the key 169, and the ball nut 162 is connected to therack shaft 106 by means of the ball screw mechanism 166. Accordingly,the driven bevel gear 161, the ball nut 162, and the rack shaft 106 arecoaxial.

[0059] As shown in FIG. 7, the driven bevel gear 161 includes a ringportion 611 having teeth and a boss portion 612, contacts the key 169. Adamper 615 composed of an elastic material such as rubber is arrangedbetween the ring portion 611 and the boss portion 612. The cylindricaldamper 615 is press-fitted between the ring portion 611 and the bossportion 612, or is formed by adhering an elastic body throughvulcanization. The damper 615 serves to reduce the rattling noisegenerated by the bevel gears 111 and 161 and absorb impacts applied inthe axial direction of the rack shaft 106 or around the bevel gearmechanism due to, for example, reverse input from the steered wheels.

[0060] As shown in FIG. 7, a collar 115 is attached to the shaft of thedrive bevel gear 111, and a collar 165 is arranged adjacent to thedriven bevel gear 161. The thicknesses of the collars 115 and 165 areadjusted such that the intersecting point X of the axis O of the drivebevel gear 111 (rotational centerline of the motor 103) and the axis Oof the driven bevel gear 161 (rotational centerline of the rack shaft106) is at a predetermined position on the axis of the rack shaft 106and the backlash between the bevel gears 111 and 161 is set to anoptimum value. When the prepared collars 115 and 165 have differentthicknesses, the backlash between the collars 115 and 165 is always setat an optimum value regardless of differences between production lots.

[0061] The power steering apparatus 100 of the second embodiment has theadvantages described below.

[0062] (11) Since the transmission mechanism 101 includes the angularbevel gear mechanism and backlash adjustment or the like is performedrelatively easily using the collars 115 and 116, the power steeringapparatus 100 is suitable for mass production.

[0063] (12) Since the bevel gear mechanism can perform reversible ortwo-way power transmission, even when the motor 103 fails to function orstops functioning, the steering operation may be performed manually withthe pinion gear 191 and the steering shaft. Accordingly, the powersteering apparatus 100 has a fail-safe function.

[0064] (13) The rotational axis M of the motor 103 intersects the axis Oof the rack shaft 106 at point X, and the axis M of the motor 103 andthe axis O of the rack shaft 106 lie on the same plane. That is, themotor 103 is installed such that the motor 103 is not offset from therack shaft 106. As a result, the space around the power-assist unit 105is used effectively.

[0065] (14) Since the power-assist unit 105 and the pinion gear 191 ofthe steering gear mechanism 109 are arranged symmetrically to eachother, many parts of the power steering apparatus 100 can be used incommon for right-hand drive vehicles and left-hand drive vehicles. Inaddition, even when the vehicle must be converted, for example, fromleft-hand drive to right-hand drive in accordance with where the vehicleis shipped to, the necessary changes can be made quickly.

[0066] (15) In the electric power steering apparatus 100 of the secondembodiment, the transmission mechanism 101 is formed by a bevel gearmechanism. Since the power transmission efficiency of the bevel gearmechanisms is significantly superior to that of worm wheel mechanisms,the power of the motor 103 is efficiently transmitted to the rack shaft106. As a result, a relatively compact motor may be used as the motor103. In addition, although the distance between the axis of the worm andthat of the wheel must be accurately maintained in worm wheelmechanisms, the restriction on the distance between the axes isrelatively mild in bevel gear mechanisms.

[0067] (16) In the driven bevel gear 161 attached to the ball nut 162,the damper 615 composed of a rubber-like elastic material is provided onthe boss portion 612. The damper 615 reduces abnormal noise generated bythe power-assist unit 105, for example, the rattling noise generated bythe bevel gear mechanism, which forms the transmission mechanism 101. Inaddition, when an impact force is applied in the axial direction of therack shaft 106 or around the bevel gear mechanism due to, for example,reverse input from the steered wheels, the impact force is reduced bythe damper 615.

[0068] (17) In the second embodiment, the driven bevel gear 161 isintegrated with the ball nut 162, and the ball nut 162 is arrangedcoaxially with the rack shaft 106 by means of the ball screw mechanism166. Accordingly, the power (assist force) of the motor 103 istransmitted to the rack shaft 106 via the ball screw mechanism 166, thefrictional resistance of which is low, and the assist force of the motor103 is smoothly transmitted to the steering gear mechanism 109.

[0069] (18) Since the transmission mechanism 101 is an angular bevelgear mechanism, the angle between the central axis of the drive bevelgear 111 and that of the driven bevel gear 161 can be set to any angleexcept a right angle. Accordingly, the intersection angle between theaxis M of the motor 103 connected to the transmission mechanism 101 andthe axis O of the rack shaft 106 may be set with a large degree offreedom and other devices can be easily arranged around the electricpower steering apparatus 100.

[0070] When the transmission mechanism 101 includes a miter gearmechanism, the intersection angle between the central axes of the twogears is limited to a right angle. However, the advantages describedabove in items (11) to (17) are obtained.

[0071] Next, an electric power steering apparatus 200 according to athird embodiment of the present invention will be described.

[0072] As shown in FIG. 8, the power steering apparatus 200 includes arack-and-pinion steering gear mechanism 209 and a power-assist unit 208having an electric motor 203. The steering gear mechanism 209 includes asteering shaft (not shown) and a pinion gear 291, which is connected toa rack shaft 206.

[0073] The pinion gear 291 and the power-assist unit 208 are arranged atdifferent positions. More specifically, the pinion gear 291 and thepower-assist unit 208 are separated from each other along the axis ofthe rack shaft 206 and lie along the same plane in a generally symmetricmanner. Since the power-assist unit 208 is separated from the piniongear 291, the power steering apparatus 200 is well-balanced. Inaddition, the structure around the pinion gear 291, where variouscomponents related to the steering gear mechanism 209 are arranged in aconcentrated manner, may be simplified. Thus, the space around thesteering gear mechanism 209 may be used effectively. In addition, whenthe installation position of the steering wheel is changed between theleft and right due to a change in where the vehicle is shipped to or thelike, the necessary changes in the power steering apparatus 200 may bemade quickly.

[0074] As shown in FIG. 10, a rotating shaft 231 of the electric motor203 is connected to a rotating shaft 211 of a drive gear (small screwgear) 201 by a coupling 202. The drive gear 201 meshes with a drivengear (screw gear) 261, which is formed integrally with a ball nut 262.The drive gear 201 and the driven gear 261 define a reduction mechanism.A casing 235 of the electric motor 203 is fixed to a housing 215, whichaccommodates the small screw gear 201 and the coupling 202, by means ofan elastic member 205 having a predetermined thickness.

[0075] Next, the power-assist unit 208 will be described below withreference to FIG. 10. As shown in FIG. 10, the rotating shaft 231 of theelectric motor 203 is connected to the rotating shaft 211 of the smallscrew gear 201 by the Oldham coupling 202. The elastic member 205, whichhas the predetermined thickness, is arranged between a flange 355 of thecasing 235, which accommodates the electric motor 203, and a flange 245of the housing 215, which accommodates the small screw gear 201. Theflanges 245 and 355 are fixed to each other by fastening means 55 suchas bolts. Since the elastic member 205 is arranged between the housing215 and the electric motor 203, there is a risk that the axis of therotating shaft 231 of the electric motor 203 and that of the rotatingshaft 211 of the small screw gear 201 will be misaligned duringoperation of the electric motor 203. The Oldham coupling 202 serves toabsorb the influence of misalignment of the axes.

[0076] As shown in FIG. 11, a flexible coupling 202 a composed of metalmay also be used in place of the Oldham coupling 202. The flexiblecoupling 202 a is formed by a plurality of flexible metal rings(metal-ring aggregate) arranged between the motor rotating shaft 231 andthe rotating shaft 211 of the small screw gear 201. Since the metalrings can be inclined, although only slightly, with respect to oneanother, the entire body of the flexible coupling 202 a may be deformedby a predetermined amount. Accordingly, even when the axis of the motorrotating shaft 231 and that of the rotating shaft 211 of the small screwgear 201 are misaligned or tilted with respect to each other, theflexible coupling 202 a can cope with such conditions.

[0077] Next, the connection between the casing 235 and the housing 215will be described. As shown in FIG. 12, the elastic member 205, whichhas the predetermined thickness, is arranged between the flange 355 ofthe casing 235 and the flange 245 of the housing 215. The elastic member205 is in contact with the entire surfaces of the flanges 245 and 355.The elastic member 205 has a collar-receiving hole 511. A collar 251having an outer diameter that is substantially the same as the innerdiameter of the collar-receiving hole 511 is fitted in thecollar-receiving hole 511. The flange 355 has a collar-receiving hole356 having an inner diameter that is slightly larger than the outerdiameter of the collar 251. The collar 251 is inserted in thecollar-receiving hole 356. The lower end of the collar 251 is in contactwith the upper surface of the flange 245. The collar 251 restricts thedistance between the flanges 245 and 355. A flanged bolt 55 is insertedthrough the collar 251 to fasten the flanges 245 and 355. When theelastic member 205 is deformed, the collar 251 and the bolt 55 toleratestilting of the casing 235.

[0078] Next, a buffer mechanism for reducing an impact applied at theends of the steering stroke of the power steering apparatus 200 when therack shaft 206 reaches the ends of its stroke will be described.

[0079] The power steering apparatus 200 includes a buffer mechanismwhich prevents a ball joint 265 formed on each end of the rack shaft 206from hitting its respective stopper 257 of the housing 215 andgenerating a metallic impact noise when the rack shaft 206 reaches theends of its stroke. As shown in FIGS. 9 and 10, the buffer mechanismincludes a buffer member 207, like a leaf spring or a belleville spring,which is adjacent to the stopper 257 formed on part of the housing 215.When the rack shaft 206 reaches the end of its stroke, the buffer member207 contacts the ball joint 265. Accordingly, the stopper 257 isprevented from hitting the ball joint 265 and generating vibration andnoise.

[0080] The power steering apparatus 200 of the third embodiment has theadvantages described below.

[0081] (19) The casing 235 is connected to the housing 215 by means ofthe elastic member 205. The vibration and noise of the electric motor203 are blocked by the elastic member 205 and are not transmitted to thepassenger compartment.

[0082] (20) The rotating shaft 231 of the electric motor 203 isconnected to the rotating shaft 211 of the small screw gear 201 by theOldham coupling 202 or the flexible coupling 202 a. Accordingly, evenwhen the elastic member 205 is deformed and the central axes of the twoshafts 211 and 231 are misaligned, the rotational driving force of theelectric motor 203 is smoothly transmitted to the small screw gear 201via the coupling 202 or 202 a. The vibration damping characteristics ofthe elastic member 205 are easily adjusted by selecting the properthickness or hardness of the elastic member 205.

[0083] (21) The power-assist unit 208 and the pinion gear 291 arearranged at different positions. More specifically, the power-assistunit 208 and the pinion gear 291 are arranged on the axis of the rackshaft 206 at generally symmetric positions. Accordingly, the structurearound the pinion gear 291, where various components related to thesteering gear mechanism 209 are arranged in a concentrated manner, issimplified, and the space around the steering gear mechanism 209 may beused effectively. In addition, the adjustment of the gear mechanism 209may be performed smoothly.

[0084] (22) The buffer member 207 is adjacent to the stopper 257 of thehousing 215, which accommodates the rack shaft 206. When the rack shaft206 reaches the end of its stroke, the buffer member 207 contacts theball joint 265 provided at each end of the rack shaft 206 so that theball joint 265 does not hit the stopper 257. This prevents thegeneration of vibration and noise. In addition, the ball nut 61 isprevented from being caught in the ball screw mechanism 266.

[0085] (23) The power-assist unit 208 may be arranged at any angle in arange of 360 degrees about the axis of the rack shaft 206. Thus, thepower-assist unit 208 may be installed with a large degree of freedom.

[0086] (24) Even when the central axes of the shafts 211 and 231 aremisaligned due to the deformation of the elastic member 205, themisalignment is tolerated since an Oldham coupling is used as thecoupling 202.

[0087] The elastic member 205 may also be used in the power steeringapparatuses 11 and 100 of the first and second embodiments.

1. An electric power steering apparatus (11; 100; 200) for transmittinga driving force of a motor (28; 103; 203) to a rack shaft (18; 106; 206)via a reduction mechanism (G; 101; 201 and 261) and a ball screwmechanism (21) to assist movement of the rack shaft in a longitudinalaxis (O) thereof, the electric power steering apparatus beingcharacterized in that: the reduction mechanism includes a drive gear(29; 111; 201) connected to a rotating shaft (28 a; 231) of the motor,and a driven gear (16 a; 161; 261) meshed with the drive gear andconnected to the ball screw mechanism, with the rotating shaft of themotor intersecting the longitudinal axis of the rack shaft at anarbitrary angle (θ2).
 2. The electric power steering apparatus accordingto claim 1, characterized in that the drive gear is a first screw gear,the driven gear is a second screw gear, and the reduction mechanism is ascrew gear mechanism (101) including the first screw gear and the secondscrew gear.
 3. The electric power steering apparatus according to claim1, characterized in that the drive gear is a first bevel gear (111), thedriven gear is a second bevel gear (161), and the reduction mechanism isa bevel gear mechanism including the first bevel gear and the secondbevel gear.
 4. The electric power steering apparatus according to claim3, characterized in that each of the first bevel gear and the secondbevel gear is an angular bevel gear, and the bevel gear mechanism is anangular bevel gear mechanism.
 5. The electric power steering apparatusaccording to any one of claims 1 to 4, characterized in that the ballscrew mechanism includes a ball screw nut (20) having a helical firstball groove (17 a) extending along its inner circumferential surface, ahelical second ball groove (18 a) extending along an outercircumferential surface of the rack shaft, and a plurality of balls (19)arranged between the helical first ball groove and the helical secondball groove, and the driven gear is provided on the outercircumferential surface of the ball screw nut.
 6. The electric powersteering apparatus according to claim 5, characterized in that thedriven gear is formed on the outer circumferential surface of the ballscrew nut.
 7. The electric power steering apparatus according to claim5, characterized in that the driven gear is connected to the ball screwnut by a damper (615), which is an elastic body.
 8. The electric powersteering apparatus according to one of claims 1 to 4, characterized by:a coupling (202) arranged between the rotating shaft (231) of the motorand a rotating shaft (211) of the drive gear (201) to transmit therotation of the rotating shaft (231) of the motor to the rotating shaft(211) of the drive gear, wherein the coupling tolerates axesmisalignment of the rotating shaft (211) of the drive gear and therotating shaft (231) of the motor; a casing (235) for accommodating themotor; a housing (215) for accommodating the drive gear; and an elasticmember (205) having a predetermined thickness arranged between thecasing and the housing, with the casing attached to the housing by meansof the elastic member.
 9. The electric power steering apparatusaccording to claim 8, characterized in that the coupling is an Oldhamcoupling.
 10. The electric power steering apparatus according to any oneof claims 1 to 4, further characterized by a rack housing (13)accommodating the rack shaft, wherein the motor is attached to the rackhousing (13) in the vicinity of one of the ends of the rack housing(13).
 11. The electric power steering apparatus according to claim 10,further characterized by a pinion shaft (25) for transmitting a drivingforce of a steering wheel to the rack shaft, the pinion shaft includinga pinion gear (25 a) meshed with the rack teeth (18 b) formed on therack shaft, and the pinion shaft being arranged near the other one ofthe ends of the rack housing.
 12. The electric power steering apparatusaccording to claim 11, characterized in that the axis (M) of the motorand the axis (P) of the pinion shaft are symmetric to each other about areference line (T) perpendicular to a longitudinal axis of the rackhousing at the center (S) of the rack housing along the longitudinalaxis.
 13. The electric power steering apparatus according to claim 12,characterized in that the motor and the pinion shaft become farther fromthe reference line (T) as the motor and the pinion shaft become fartherfrom the rack housing.
 14. An electric power steering apparatus (11;100; 200) characterized by: a rack shaft (18; 106; 206) having alongitudinal axis (O); a rack housing (16) for accommodating the rackshaft and having a first end and a second end; a pinion shaft (25)arranged in the vicinity of the first end of the rack housing andconnected to the rack shaft; a motor (28; 103; 203) arranged in thevicinity of the second end of the rack housing; a ball nut (20; 162;262) arranged coaxially to the rack shaft in a manner covering the rackshaft, wherein the ball nut is rotated relative to the rack shaft tomove the rack shaft along the longitudinal axis; and a reductionmechanism (G; 101; 201 and 261) arranged between the motor and the ballnut to transmit rotation of a rotating shaft (28 a) of the motor to theball screw nut while reducing the speed of the rotation, wherein thereduction mechanism includes a drive gear (29; 101; 201) coaxiallyconnected to the rotating shaft of the motor and a driven gear (16 a;161; 261) arranged on the outer surface of the ball screw nut and meshedwith the drive gear, with the axis of the drive gear inclined withrespect to the axis of the driven gear.
 15. The electric power steeringapparatus according to claim 14, characterized in that the driven gearis connected to the ball nut by a damper (615) such that the driven gearis coaxial with the rack shaft.